To see all the greyed out options to be enabled first you need to add the Show column grand totals or row grand totals, then the Total all using options like (sum, avg,minimum, maximum) options will be enabled like below:

A table calc is already an aggregation of an aggregation, when dragging a table calc into a view the option to change the aggregation (i.e. count, sum, min, etc) isn't provided, I suspect this is also the reason you're also not able to Total this separately

You might be able to use a nested table calculation in order to get the desired result, however Tableau's grand totals and subtotals are computed as separate calculations at different level of detail. So we have to simultaneously solve each table calculation for multiple levels of detail and that can get very tricky.